If you aren't paying for a product, then you're the product being sold. Web site "How Do They Make Money?" examines a number of tech companies, telling you how they make money—and which ones might be tracking you to do it.

Everybody wants stuff for free but we seldom question what it means to get something for free.…
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Some of these you may know already: Dropbox makes money by having a premium service they offer to their users, while Facebook and Twitter make money solely through advertising. It's an interesting look at how your favorite tech services make money, though, and a good way to find out who's relying on advertising the most—and thus who might be trying to track you on the web. You can filter the list by revenue type or service type too, which is handy. We only wish it was a bigger list, so we could see more companies that are trying to track and sell your data. Hit the link to see the list.